1. Field of the Invention
The present invention relates to a snubber circuit used to reduce stress on a switching transistor and to increase overall efficiency of a switched mode power supply.
2. Description of Related Art
Computer systems are getting smaller and more complex. DC converters or switched mode power supplies are used to provide power to computer systems since the high frequencies require smaller and lighter power transformers and filtering components. In general, a switched mode power supply converts ac voltage to a dc source voltage, which is connected to one terminal of the primary inductance of a transformer. The other terminal of the primary inductance is connected to a transistor switch which provides a conductive path back to the return of the dc source voltage. The transistor switch controls the current through the primary inductance of the transformer. When the transistor switch is turned on, the switch is closed so that current can flow from the dc source voltage through the primary inductance of the transformer and through the transistor switch and back to the return of the dc source voltage. When the transistor switch is turned off, the switch is opened so that it interrupts the current flow through the transistor switch and primary inductance of the transformer. Energy from the current flow is transferred through the transformer to the secondary inductance to the output circuit which provides a regulated voltage source at the output of the power supply. A pulse width modulator (PWM) circuit monitors the output voltage through a feedback circuit and generates the activation signal which is connected to the control terminal of the transistor switch to turn it on and off as required. The activation signal is a square wave, the frequency of which is determined by an internal oscillator of the PWM. When the signal goes high the transistor switch turns on allowing current flow, and when the signal goes low, the transistor switch turns off. The time from turn on to turn off in each cycle is referred to as the pulse width. If the output voltage level begins to drop, the PWM circuit increases the width of each pulse, thereby turning the transistor switch on longer in a given cycle, which allows greater power to be transferred to the output circuit which eventually increases the output voltage to return it back to the proper level.
The transistor switch undergoes stress while switching since the transformer inductances resist any changes in current. As the transistor is turning off, the resistance of the conductive path through the transistor increases rapidly until the conductive path is essentially cut off. As the conductive path through the transistor is being cut off, the current through the primary inductance must be diverted away from the transistor switch and eliminated before the next cycle begins. A snubber circuit diverts the current away from the switching transistor and absorbs this extraneous energy. The snubber circuit of prior art diverts this current through a resistor which converts the energy into unwanted heat. This heat energy must be dissipated quickly to prevent the power supply from overheating. The resistors and the components used in prior art to dissipate the heat energy consume valuable space and increase the cost of the power supply. Furthermore, this energy converted to heat is lost, thereby reducing the overall efficiency.
When the switching transistor is turning off, a voltage spike appears across the primary inductance due to leakage inductance, which creates stress on the transistor which could destroy the transistor if the voltage surpasses its maximum voltage rating. A clamp circuit reduces the peak voltage of the voltage spike by clamping the voltage across the primary inductance to the dc source voltage level. A lossy clamp would convert the energy in this voltage spike to heat by placing the voltage across a resistor. This added heat is not desirable and further decreases the efficiency of the power supply.